Question about the sand on the reefs?

[Brules]

Ok so all joking aside, we have all seen pics, some have seen first hand the sand that was deposited on the Cozumel Reefs. I will be seeing it first hand in July (cant wait!) but I have a serious question:

Q - How do/does the reefs deal with the amount of sand deposited on the reefs? Will the water just blow it off over time? Will the reefs absorb it and gorw new growth on it? Will critters pick it off or somehow remove it?

Q - Has something like this happened on this scale somewhere else? If so where and what was the recovery time? (Caymen after their bad hurricane a few years ago?)

Q - Does anyone know/think/guess whether or not all the man made beaches on the west side (read: trucked in sand for the iron shore) made the bad situation worse, by providing the hurricane *ammunition* to damage the reefs by depositing sand that normally would not be there on the reefs?

Q - I have seen alot of talk about the sand blasting the reefs took, that is being mistaken as *bleaching*. Can the reefs re-grow the parts sand blasted off, where as with bleaching they usually die and do not re-grow?


Please understand these Q's are not meant to provoke/instigate any large war of the words like some previous Q's have. They are just questions with a logical bassis that I am interested in knowing about. I am fascinated with the reefs and how they are actually alive and how the critters interact/depend on them....so I am curious as to what happens now after the hurricane.

No, the island is not destroyed, the reefs are not destroyed, but both did sustain some damage and I am interested in hearing about how it might recover/repair itself.

So what does everyone think/know/guestimate?



PS - Is anyone down there (Christi?!?!) documenting the reefs with pics over time to see how they are recovering/cleaning themselves off/regrowing? It sure would be cool to see some camera-inclined SB'ers document a few popular dive sites over time with pics that you could compare. Would be fascinating ot see the changes!

 

[archman]

Q - How do/does the reefs deal with the amount of sand deposited on the reefs? Will the water just blow it off over time? Will the reefs absorb it and gorw new growth on it? Will critters pick it off or somehow remove it?

This is simple. It kills the reef. Cozumel is blessed with interesting currents and deep canyons close to shore; these two factors help the sand get *off* the reef and out of sight. Reef recovery is directly related to amount of sediment, and exposure time. If sand persists, it'll stunt recovery. If a reef was blanketed for more than a few weeks, it's effectively dead and will have to start all over. If there's only a little bit of sand, there are certain coral species (i.e. Siderastrea) that have higher tolerances than others (Diploria). This can lead to community shift towards a lower diversity reef complex.

Q - Has something like this happened on this scale somewhere else? If so where and what was the recovery time? (Caymen after their bad hurricane a few years ago?)

This sort of thing happens all the time, just not so much at popular tourist destinations. As for recovery, if the reef is blanketed, it may not recover for decades, if ever. If there's a lot of physical damage (dislodged reef matrix), the branching acroporid corals are toast, and will take at least a decade to suitably "look nice and big" again. It's common for fire corals and poritid corals to reoccupy these shallow forereef zones, and those things aren't nearly as pretty.

Cozumel possesses key hydrographic features that make it somewhat sheltered in terms of ecosystem stability. It'll "do better" in post-hurricane recovery than most other Caribbean reefs would.

Q - Does anyone know/think/guess whether or not all the man made beaches on the west side (read: trucked in sand for the iron shore) made the bad situation worse, by providing the hurricane *ammunition* to damage the reefs by depositing sand that normally would not be there on the reefs?

Yes, any large source of soft sediments adjacent to a coral reef is a potential storm risk. Most corals HATE sand in the water column.

Q - I have seen alot of talk about the sand blasting the reefs took, that is being mistaken as *bleaching*. Can the reefs re-grow the parts sand blasted off, where as with bleaching they usually die and do not re-grow?

Sand scour is far more deadly than bleaching, make no mistake. First off, it kills the coral... utterly. Then it opens up the bioherm to initial colonization by fast-growing seaweeds, encrusting tunicates & sponges, and mat-forming anemones. Not corals. Corals may eventually resettle and start to "retake" the reef complex, but it takes them a long time to do so. An algal reef, conversely, can jump-start in under five years.

Many of the Cozumel reefs do not appear to have been sand-scoured, however. Nor were they significantly bleached. If you want to see bleached reefs, visit the eastern caribbean. For sand-scoured reefs, visit parts of Florida.

 

[Divingnthedark]

Thanks for the thorough explanation Archman...I do hope fire corals don't move in...I thought that on some divesites in Belize it seemed abundant. I've seen sea fans and conch shells covered in the stuff. One fire coral covered conch shell I got a pic of had a giant hermit crab in it! Talk about defense! =)

 

[Brules]

Wow that was better than I expected!!!!! It is absolutely fascinating just how *alive* the reefs actually are!!!

Thanks again!

 

[MikeGT]

Great JOB!!!

Mike (Another Science Teacher)

PS Go to www.springerlink.com for an interesting abstract on species diversity and hurricanes

 

[cjames]

When I was there in March, I took some fairly detailed looks and pictures of the reefs in about 5 spots. Clearly the sand is causing some die off on parts of the reef where it is flat and only strong current or wave action can clear it and as archman said, other life is taking hold where the small polyp stony corals are dying off. I noticed gorgonians and other soft corals starting to grow on a couple of heads. I also took measurements of both soft and hard corals in a couple of places and am going to remeasure when i go back in May to see how things are regrowing. Most SPS corals grow very slowly, but I measured down to the millimeter so I hope to see some growth.

Another intesting thing was that while the corals got clobbered by the sand, a lot of the sponge life seemed to still be ok under the sand. I guess that makes sense becuase it is not photosynthetic.

 

[archman]

Another intesting thing was that while the corals got clobbered by the sand, a lot of the sponge life seemed to still be ok under the sand. I guess that makes sense becuase it is not photosynthetic.

Many Caribbean sponges do in fact harbor photosynthetic algae, it's just more poorly studied. Many gorgonians also have algae, and a slough of other invertebrates (anemones, hydroids, tunicates, nudibranchs, clams, jellies).

But the hermatypic corals have the highest densities, and can't survive for long without their seaweed helpers. Most of the other weed-harboring critters can survive without them, though they'll not *like it*.

Unfortunately, it's the hermatypic corals that create coral reefs. Without them, everything else eventually falls apart. Fire corals and poritid "finger" corals don't rate as true hermatypics, as they don't contribute significantly to bioherm construction. So takeovers by these species (or sponges, gorgonians, etc.) is not true replacement, but signals a (non-sustainable) habitat shift.

Fire coral "reefs" suck. There are a bunch of those throughout the Bahamas, usually overlying recently dead hermatypic reefs.

 

[PaulChristenson]

Fire coral "reefs" suck.

Maybe it will teach new divers to stay off the reef...

 

[Brules]

So let me ask this.....what keeps the local dive community (scientists/shop owners/dm's etc) from helping the reef out by faning it off/blowing it off (the sand)?

I know it is best to let nature take care of itself, but we (humans) mess with it all the time already.....? I mean, if say something like this caused a major die off, wouldnt it be prudent to do as much as possible to ensure its survival?

We try to save beached whales, we fight forest fires...whats the difference?

 

[archman]

So let me ask this.....what keeps the local dive community (scientists/shop owners/dm's etc) from helping the reef out by faning it off/blowing it off (the sand)?

Weeeellll... it's a TON of work. And it's not as easy as it looks. Still, one might occasionally see some conscientious DM or instructor brushing off sand from the odd coral head. It's the ecological equivalent to picking up a piece of trash while you're walking to your car.

There's also a problem with fanning too hard. Sometimes it's better to leave a little bit of sand on the reef, rather than blast it off with some overcharged fanning. *That* might strip the coral's protective mucous sheet off. Corals hate that almost as much as being buried. Shallow branching corals like Acropora live in high current/wave swash zones, and are specially adapted for losing their goo (they overproduce the stuff). So feel to swish sand off those things as much as you want!